Might underwear widen on vacation? (5)
I believe the answer is:
brawn
'might' is the definition.
(thesaurus)
'underwear widen on vacation?' is the wordplay.
'underwear' becomes 'bra' (I've seen this before).
'on vacation?' suggests removing the centre (I've seen 'on vacation' mean this).
'widen' with its middle taken out is 'wn'.
'bra'+'wn'='BRAWN'
(Other definitions for brawn that I've seen before include "Physical strength; cooked meat" , "Jellied meat -- not brain!" , "meatloaf" , "Muscle; strength" , "Jellied meat - muscle" .)